Secondary battery and electronic device

Abstract

Provided are a secondary battery and an electronic device. The secondary battery comprises an electrode assembly. The electrode assembly comprises a negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector and negative electrode active material layers arranged on surfaces of the negative electrode current collector. The negative electrode current collector comprises a first area, and the negative electrode active material layers are arranged on two opposite surfaces of the first area. The total thickness of the negative electrode active material layers on the first area is T, wherein 0.07 mm≤T≤0.15 mm. A plurality of recesses are formed on the negative electrode active material layers. The compaction density of the negative electrode active material layers is D (g / cm3), wherein 3T+1≤D≤8T+1.

Description

Secondary batteries and electronic devices Technical Field

[0001] This application relates to the field of energy storage technology, and more particularly to a secondary battery and an electronic device having said secondary battery. Background Technology

[0002] Secondary batteries (such as lithium-ion secondary batteries) are widely used in electronic mobile devices, power tools and electric vehicles, and people have increasingly higher requirements for the performance of secondary batteries.

[0003] In related technologies, increasing the thickness of the active material layer on the electrode is used to improve the energy density of secondary batteries. However, the design of thick electrodes prolongs the migration path of lithium ions and increases migration resistance. During cycling, lithium ions have difficulty quickly reaching the active material layer adjacent to the current collector, leading to increased concentration polarization and problems such as reduced kinetic performance and cycle capacity decay. Therefore, for secondary batteries with thick electrodes, how to achieve both high energy density and excellent kinetic and cycle performance has become an urgent problem to be solved.

[0004] Summary of the Invention

[0005] This application provides a secondary battery that can achieve both high energy density and superior kinetic and cycle performance, as well as an electronic device having the aforementioned secondary battery.

[0006] This application provides a secondary battery, including an electrode assembly. The electrode assembly includes a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector. The negative current collector includes a first region, and negative active material layers are disposed on both opposite surfaces of the first region. The total thickness of the negative active material layer on the first region is T, where 0.07 mm ≤ T ≤ 0.15 mm. The negative active material layer has multiple recesses, and the compaction density of the negative active material layer is D, in g / cm³. 3 , 3T+1≤D≤8T+1.

[0007] This application targets negative electrode sheets with a relatively thick negative electrode active material layer. By creating recesses in the negative electrode active layer, the wettability of the electrolyte to the negative electrode sheet is improved, reducing ion migration resistance and facilitating the transport and intercalation of lithium ions within the negative electrode active material layer. This addresses issues such as poor kinetic performance and low cycle capacity decay caused by the increased thickness of the negative electrode active material layer. Simultaneously, by setting the relationship between compaction density and total thickness, on the one hand, the secondary battery can maintain a high energy density; on the other hand, it ensures close contact between the negative electrode active particles, providing an efficient ion transport path and reducing impedance, thereby further improving the kinetic performance of the secondary battery. Furthermore, it constructs more pores within the negative electrode active material layer to further improve electrolyte wettability, thereby reducing the risk of partial loss of negative electrode active material and lithium plating within the recesses due to the recesses, further improving the cycle performance of the secondary battery. Therefore, by incorporating recesses in the thick electrode sheet and further defining the relationship between compaction density and total thickness, this application achieves a balance between the energy density, kinetic performance, and cycle performance of the secondary battery, enabling it to achieve both high energy density and superior kinetic and cycle performance. Furthermore, by defining the relationship between compaction density and total thickness, the risk of negative electrode active material detachment (recessed sidewall collapse) when recesses are incorporated is reduced. Simultaneously, the risk of excessive fresh surface area generation and electrolyte consumption by negative electrode active material particles when recesses are incorporated is reduced, thereby mitigating the risk of increased side reactions at the recessed sidewalls and exacerbating lithium plating within the recesses.

[0008] Based on the first aspect, in some possible implementations, 3.5T+1≤D≤6.5T+1. This can further improve the energy density of the secondary battery while ensuring close contact between the negative electrode active particles and constructing more pores in the negative electrode active material layer to further improve the electrolyte wetting degree, thereby further improving the kinetic performance and cycle performance of the secondary battery.

[0009] Based on the first aspect, in some possible implementations, 4.5T+1≤D≤5.5T+1. This can further improve the energy density of the secondary battery while ensuring close contact between the negative electrode active particles and constructing more pores in the negative electrode active material layer to further improve the electrolyte wetting degree, thereby further improving the kinetic performance and cycle performance of the secondary battery.

[0010] Based on the first aspect, among some possible implementations, 1.2 g / cm 3 ≤D≤2.2g / cm 3 This allows secondary batteries to achieve both high energy density and superior kinetic and cycle performance.

[0011] Based on the first aspect, among some possible implementations, 1.3 g / cm 3≤D≤1.9g / cm 3 This allows secondary batteries to achieve both higher energy density and better kinetic and cycle performance.

[0012] Based on the first aspect, in some possible implementations, the electrode assembly is a wound structure, comprising a straight section and a curved section connected along the winding direction. The negative electrode active material layer includes a first active material region located in the straight section and a second active material region located in the curved section. The compaction density of the first active material region is D1, and the compaction density of the second active material region is D2, 1.3 g / cm³. 3 ≤D1≤2.0g / cm3, 1.2g / cm 3 ≤D2≤1.5g / cm3. This can improve the problem of lithium deposition caused by compression and high internal stress of the electrode in the curved section of the second active material region. At the same time, the compaction density of the first active material region in the straight section is high. Without significantly deteriorating the kinetic performance and lithium deposition problem in the first active material region, the first active material region can play a greater capacity role. Therefore, the energy density of the secondary battery is further improved.

[0013] Based on the first aspect, in some possible implementations, the electrode assembly is a stacked structure. The negative electrode active material layer includes a third active material region and a fourth active material region disposed around the outer periphery of the third active material region. The compaction density of the third active material region is D3, and the compaction density of the fourth active material region is D4, both 1.2 g / cm³. 3 ≤D3≤2.0g / cm3, 1.2g / cm 3 ≤D4≤1.7g / cm3. This can improve the problem of lithium plating in the fourth active material region located at the edge due to its relatively thin thickness and relatively small amount of negative electrode active material. At the same time, the third active material region located in the center has a high compaction density. Without significantly deteriorating the kinetic performance and lithium plating problem in the third active material region, the third active material region can play a greater capacity role. Therefore, the energy density of the secondary battery is further improved.

[0014] Based on the first aspect, in some possible implementations, the width of the recess is 70 μm to 100 μm, the depth is 5 μm to 100 μm, and the center-to-center distance between two adjacent recesses is 1 mm to 2 mm. Appropriately sized recesses can reduce the risk of excessive loss of negative electrode active material, which is beneficial for maintaining high energy density and reducing the risk of lithium plating degradation within the recesses. They can also suppress lithium ion accumulation within the recesses and maintain the structural stability of the negative electrode sheet to reduce electrode assembly deformation. This further improves the problem of easy lithium plating within the recesses after creating recesses on thick electrodes, thus improving the cycle performance of the secondary battery. Appropriate recess distribution density can improve electrolyte wetting, reduce ion migration resistance, and improve the problem of easy lithium plating within the recesses, thereby improving the kinetic and cycle performance of the secondary battery.

[0015] Based on the first aspect, in some possible implementations, the recess is circular or square when viewed from the thickness direction of the negative electrode sheet. A circular recess facilitates the uniform diffusion of lithium ions within the negative electrode active material layer, improves electrolyte wetting, and reduces ion migration resistance, thus mitigating problems such as poor kinetic performance and low cycle capacity decay caused by the increased thickness of the negative electrode active material layer. A square recess increases the contact area between the electrolyte and the negative electrode active material layer at the boundary, which helps to increase the diffusion rate of lithium ions within the negative electrode active material layer, thereby further improving problems such as poor kinetic performance and low cycle capacity decay caused by the increased thickness of the negative electrode active material layer.

[0016] Based on the first aspect, in some possible implementations, the CB value of the secondary battery is 1 to 1.05, thereby further reducing the risk of lithium plating in the negative electrode active material layer and improving the cycle performance of the secondary battery.

[0017] Based on the first aspect, in some possible implementations, the recess is obtained by removing part of the negative electrode active material layer.

[0018] A second aspect of this application also provides an electronic device comprising a battery compartment and a secondary battery as described above disposed within the battery compartment. The electronic device is powered by the secondary battery, which achieves a balance between high energy density and superior kinetic and cycle performance. Attached Figure Description

[0019] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0020] Figure 1 is a schematic diagram of the structure of a secondary battery provided in one embodiment of this application.

[0021] Figure 2 is a partial enlarged view of the negative electrode plate at point II of the secondary battery shown in Figure 1.

[0022] Figure 3 is a planar schematic diagram of the negative electrode sheet shown in Figure 2 in some embodiments.

[0023] Figure 4 is a planar schematic diagram of the negative electrode sheet shown in Figure 2 in some other embodiments.

[0024] Figure 5 is a schematic diagram of the structure of a secondary battery provided in another embodiment of this application.

[0025] Figure 6 is a schematic diagram of the structure of an electronic device according to an embodiment of this application.

[0026] Key Component Symbols: Electronic Device 1, Electrode Assembly 20, Straight Section 20A, Bending Section 20B, Negative Electrode 21, Positive Electrode 22, Separator 23, Secondary Battery 100, Battery Compartment 101, Positive Current Collector 220, Positive Active Material Layer 221, Negative Current Collector 210, Negative Active Material Layer 211, First Region 2100, Recess 2110, First Active Material Region 2111, Second Active Material Region 2112, Third Active Material Region 2113, Fourth Active Material Region 2114, Winding Center Axis C, Winding Direction W, Thickness T0, T1, Width w, Depth h, Center Spacing d, Thickness Direction X. The following detailed embodiments will further illustrate this application in conjunction with the above figures. Detailed Implementation

[0027] The technical solutions in the embodiments of this application are described clearly and in detail below. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0028] The embodiments of this application will be described in detail below. However, this application may be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided to provide a thorough and detailed understanding of this application to those skilled in the art.

[0029] Additionally, for brevity and clarity, the dimensions or thicknesses of various components and layers may be enlarged in the accompanying drawings. Throughout the text, the same numerical values ​​refer to the same elements. As used herein, the terms "and / or" and "and / or" include any and all combinations of one or more of the associated enumerated items. Furthermore, it should be understood that when element A is referred to as "connecting" element B, element A may be directly connected to element B, or there may be an intermediate element C and element A and element B may be indirectly connected to each other.

[0030] Furthermore, when describing the implementation of this application, the word "may" refers to "one or more implementations of this application".

[0031] The technical terms used herein are for the purpose of describing particular embodiments and are not intended to limit this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. It should be further understood that the term "comprising," as used in this specification, means the presence of the described features, values, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, values, steps, operations, elements, components, and / or combinations thereof.

[0032] Spatial terms, such as "above," may be used herein for convenience in describing the relationship between one element or feature and another element (or feature) or feature (or feature) illustrated in the figures. It should be understood that, in addition to the directions depicted in the figures, spatial terms are intended to include different orientations of the device or apparatus during use or operation. For example, if the device in the figure is flipped, an element described as "above" or "on" other elements or features would be oriented "below" or "under" other elements or features. Therefore, the exemplary term "above" can include both above and below orientations. It should be understood that although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers, and / or portions, these elements, components, regions, layers, and / or portions should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, or portion from another element, component, region, layer, or portion. Therefore, a first element, component, region, layer, or portion discussed below may be referred to as a second element, component, region, layer, or portion without departing from the teachings of the exemplary embodiments.

[0033] In this application, the design relationships of greater than, less than, or not equal to parameter values ​​need to exclude reasonable errors of the measuring equipment.

[0034] Referring to Figure 1, one embodiment of this application provides a secondary battery 100, including a casing (not shown), an electrode assembly 20, and an electrolyte (not shown). Both the electrode assembly 20 and the electrolyte are located within the casing. The casing can be a packaging bag encapsulated with an encapsulating film (such as an aluminum-plastic film), such as a pouch battery. In other embodiments, the secondary battery 100 can also be a steel-cased battery, an aluminum-cased battery, etc.

[0035] The electrode assembly 20 includes a negative electrode 21, a positive electrode 22, and a separator 23, with the separator 23 disposed between the positive electrode 22 and the negative electrode 21. As shown in Figure 1, the electrode assembly 20 can be a wound structure, formed by sequentially stacking and winding the positive electrode 22, the separator 23, and the negative electrode 21. As shown in Figure 1, the electrode assembly 20 has a winding center axis C perpendicular to the paper surface and a winding direction W around the winding center axis C. The winding direction W is the counterclockwise rotation direction around the winding center axis C, as shown in Figure 2. In other embodiments, the winding direction W can also be the clockwise rotation direction.

[0036] As shown in Figure 5, in some other embodiments, the electrode assembly 20 can also be a stacked structure, which is formed by alternating layers of positive electrode 22, separator 23 and negative electrode 21.

[0037] Positive electrode sheet

[0038] The positive electrode 22 includes a positive current collector 220 and a positive active material layer 221 disposed on the surface of the positive current collector 220. The positive current collector 220 can be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, the current collector formed by combining the aforementioned conductive foil and polymer substrate.

[0039] The positive electrode active material layer 221 includes an active material, such as at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxide, lithium-rich manganese-based material, or lithium nickel cobalt aluminum oxide.

[0040] The positive electrode active material layer 221 may also include a binder for bonding the active material particles to facilitate the formation of the film layer, and also to improve the bonding force between the positive electrode active material layer 221 and the positive electrode current collector 220. In some embodiments, the binder may include, but is not limited to, at least one of the following: polyimide, polyvinyl alcohol, sodium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyethylene oxide, ethylene oxide, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polymethyl methacrylate, polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, polyacrylonitrile, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0041] The positive electrode active material layer 221 may further comprise a conductive agent, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0042] Negative electrode sheet

[0043] The negative electrode 21 includes a negative current collector 210 and a negative active material layer 211 disposed on the surface of the negative current collector 210.

[0044] The negative electrode current collector 210 can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil or carbon-based current collector, or any composite current collector disclosed in the prior art, such as, but not limited to, the current collector formed by combining the aforementioned conductive foil and polymer substrate.

[0045] The negative electrode active material layer 211 contains an active substance, which may be selected from at least one of graphite-based materials, alloy materials, lithium metal and its alloys. The graphite-based materials may be selected from at least one of artificial graphite and natural graphite; the alloy materials may be selected from at least one of silicon, silicon oxide, tin, and titanium sulfide.

[0046] The negative electrode active material layer 211 may further include a conductive agent, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. Metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. The conductive polymer may be a polyphenylene derivative.

[0047] The negative electrode active material layer 211 may also include an adhesive, which is used to bond the negative electrode active particles to facilitate the formation of the film layer, and at the same time can improve the bonding force between the negative electrode active material layer 211 and the negative electrode current collector 210. In some embodiments, the adhesive may include, but is not limited to, polyimide, polyvinyl alcohol, sodium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyethylene oxide, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polymethyl methacrylate, polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, polyacrylonitrile, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0048] The negative electrode active material layer 211 may further include a dispersant, which is used to uniformly disperse the conductive agent and binder, thereby improving the film quality. The dispersant includes, but is not limited to, at least one of carboxymethyl cellulose salt, polyacrylate, polyethylene glycol, or polyethylene oxide. In some embodiments, the carboxymethyl cellulose salt may include at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

[0049] Referring to Figure 2, the negative electrode current collector 210 includes a first region 2100, and negative electrode active material layers 211 are provided on both opposite surfaces of the first region 2100. That is, the first region 2100 is the double-sided coating area of ​​the negative electrode current collector 210. The total thickness of the negative electrode active material layer 211 on the first region 2100 is T, where 0.07mm ≤ T ≤ 0.15mm. The negative electrode active material layer 211 has multiple recesses 2110, which can be obtained by removing part of the negative electrode active material layer 211. The method for measuring the total thickness T may include the following steps: discharging the secondary battery 100 to 3.0V, disassembling the secondary battery 100, cleaning and drying it; measuring the thickness T0 of the negative electrode sheet 21 using a micrometer; washing away the negative electrode active material layer 211 with a solvent, drying it, and measuring the thickness T1 of the negative electrode current collector 210 using a micrometer; then, calculating the total thickness T using the following formula: T = T0 - T1.

[0050] Furthermore, the compaction density of the negative electrode active material layer 211 is D, with units of g / cm³. 3 3T+1≤D≤8T+1. The method for measuring the compaction density D may include the following steps: Discharge the secondary battery 100 to 3.0V, disassemble the secondary battery 100, clean and dry it; weigh the negative electrode sheet 21 of a certain area A using a balance, and record the weight as W0; measure the thickness T0 of the negative electrode sheet 21 using a micrometer; wash off the negative electrode active material layer 211 with a solvent, dry it, measure the weight of the negative electrode current collector 210, and record it as W1; measure the thickness T1 of the negative electrode current collector 210 using a micrometer; then, calculate the compaction density D using the following formula: D=(W0-W1) / [(T0-T1)×A].

[0051] When the compaction density D is lower than 3T+1, it not only leads to a significant decrease in the energy density of the secondary battery 100, but also results in poor contact between the particles of the negative electrode active material, increased impedance, and hinders the transport of lithium ions in the negative electrode active material layer 211, leading to reduced kinetic performance. Simultaneously, the excessive porosity in the negative electrode active material layer 211 increases the contact area between the electrolyte and the negative electrode active material, easily causing side reactions and forming a solid electrolyte interphase (SEI) film on the surface of the negative electrode active material. This not only increases impedance but also exacerbates lithium plating, resulting in reduced cycle performance. Furthermore, because the particles of the negative electrode active material are relatively loose, the negative electrode active material is prone to detachment when the recess 2110 is provided, causing the sidewalls of the recess 2110 to collapse, further reducing cycle performance and even causing short circuits.

[0052] When the compaction density D is higher than 8T+1, although the energy density of the secondary battery 100 increases, the kinetic performance of the secondary battery 100 is reduced due to the excessively close contact between the particles of the negative electrode active material, which obstructs ion channels. Furthermore, the lack of pores in the negative electrode active material layer 211 for electrolyte entry leads to poor electrolyte wettability, thus exacerbating the lithium plating problem and reducing the cycle performance of the secondary battery 100. Additionally, because the particles of the negative electrode active material are in excessively close contact, when the recess 2110 is provided, the particles generate more fresh surface area and consume electrolyte, leading to increased side reactions on the sidewalls of the recess 2110. This further worsens the lithium plating problem within the recess 2110 and reduces the cycle performance of the secondary battery 100.

[0053] In this application, for a negative electrode 21 with a relatively thick negative electrode active material layer 211, a recess 2110 is provided on the negative electrode active layer. The recess 2110 can accommodate the electrolyte, which improves the wettability of the electrolyte on the negative electrode 21, reduces the ion migration resistance, and facilitates the transport and insertion of lithium ions in the entire negative electrode active material layer 211. This improves the problems of poor kinetic performance and low cycle capacity decay caused by the increased thickness of the negative electrode active material layer 211. Meanwhile, by setting the relationship between the compaction density D and the total thickness T of the negative electrode active material layer 211, the secondary battery 100 can maintain a high energy density on the one hand, and ensure close contact between the negative electrode active particles to provide an efficient ion transport path, reduce impedance, and further improve the kinetic performance of the secondary battery 100. Simultaneously, it can also construct more pores in the negative electrode active material layer 211 to further improve the electrolyte wetting degree, thereby reducing the risk of partial loss of negative electrode active material and lithium plating within the recesses 2110 caused by the recesses 2110 in the negative electrode active material layer 2111, further improving the cycle performance of the secondary battery 100. Therefore, by setting recesses 2110 in the negative electrode active material layer 211 of the thick electrode sheet and further setting the relationship between the compaction density D and the total thickness T of the negative electrode active material layer 2111, this application can achieve a balance between the energy density, kinetic performance, and cycle performance of the secondary battery 100, enabling the secondary battery 100 to achieve both high energy density and superior kinetic and cycle performance. In addition, setting the compaction density D can reduce the risk of negative electrode active material falling off (sidewall collapse of recess 2110) when recess 2110 is provided. It also reduces the risk of negative electrode active material particles generating more fresh surface and consuming electrolyte when recess 2110 is provided. Therefore, it can reduce the risk of increased side reactions at the sidewall of recess 2110 leading to deterioration of lithium plating inside recess 2110.

[0054] Furthermore, setting 3.5T+1≤D≤6.5T+1 can further improve the energy density of the secondary battery 100 while ensuring close contact between the negative electrode active particles and constructing more pores in the negative electrode active material layer 211 to further improve the electrolyte wetting degree, thereby further improving the kinetic performance and cycle performance of the secondary battery 100. Even further, setting 4.5T+1≤D≤5.5T+1 can further improve the energy density, kinetic performance, and cycle performance of the secondary battery 100.

[0055] In some embodiments, 1.2 g / cm 3 ≤D≤2.2g / cm 3 By further specifying the range of the compaction density D, the secondary battery 100 can achieve both high energy density and superior kinetic and cycle performance. Furthermore, a density of 1.3 g / cm³ can be set. 3≤D≤1.9g / cm 3 This allows the secondary battery 100 to achieve both higher energy density and better kinetic and cycle performance. For example, the compaction density can be 1.3 g / cm³. 3 1.4g / cm 3 1.5g / cm 3 1.6g / cm 3 1.7g / cm 3 Or any value within the range formed by any two of the above values.

[0056] As shown in Figure 1, when the electrode assembly 20 is a wound structure, the electrode assembly 20 may include a straight section 20A and a bent section 20B connected along the winding direction W. The negative electrode active material layer 211 includes a first active material region 2111 located in the straight section 20A and a second active material region 2112 located in the bent section 20B. The compaction density of the first active material region 2111 can be set to D1, and the compaction density of the second active material region 2112 can be set to D2, 1.3 g / cm³. 3 ≤D1≤2.0g / cm3, 1.2g / cm 3 ≤D2≤1.5g / cm3. By further setting the specific range of the compaction density of the first active material region 2111 and the second active material region 2112, the problem of easy lithium deposition caused by the compression of the second active material region 2112 and the high internal stress of the electrode at the curved section 20B can be improved. At the same time, the compaction density of the first active material region 2111 at the straight section 20A is further improved. Without significantly deteriorating the kinetic performance and lithium deposition problem at the first active material region 2111, the first active material region 2111 can play a greater capacity role, thereby further improving the energy density of the secondary battery 100.

[0057] As shown in Figure 5, when the electrode assembly 20 in other embodiments is a stacked structure, the negative electrode active material layer 211 includes a third active material region 2113 and a fourth active material region 2114 disposed around the outer periphery of the third active material region 2113. Since the negative electrode sheet 21 requires a pressing process during fabrication, the fourth active material region 2114 can extend away from the third active material region 2113 during pressing. The third active material region 2113 is surrounded by the fourth active material region 2114, restricting its movable space. Therefore, the thickness of the third active material region 2113 can be greater than the thickness of the fourth active material region 2114. Thus, the compaction density of the third active material region 2113 can be set to D3, and the compaction density of the fourth active material region 2114 can be set to D4, 1.3 g / cm³. 3 ≤D3≤2.2g / cm3, 1.2g / cm 3≤D4≤1.7g / cm3. By further specifying the compaction density range of the third active material region 2113 and the fourth active material region 2114, the problem of easy lithium plating caused by the relatively thin thickness and relatively small amount of negative electrode active material in the fourth active material region 2114 located at the edge can be improved. At the same time, the compaction density of the third active material region 2113 located in the center is further improved. Without significantly deteriorating the kinetic performance and lithium plating problem in the third active material region 2113, the third active material region 2113 can play a greater capacity role, thereby further improving the energy density of the secondary battery 100. The width of the fourth active material region 2114 can be 2mm to 10mm, for example, it can be 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm or any specific value between any two adjacent values ​​mentioned above. The width is the distance from any position of the fourth active material region 2114 away from the outer periphery of the third active material region 2113 to the inner periphery adjacent to the outer periphery of the fourth active material region 2114.

[0058] In some embodiments, the CB value of the secondary battery 100 is 1 to 1.05, thereby further reducing the risk of lithium plating in the negative electrode active material layer 211. The CB value is the ratio of the capacity per unit area of ​​the negative electrode active material layer 211 to the capacity per unit area of ​​the positive electrode active material layer 221. The method for measuring the CB value may include the following steps: 1) Let the secondary battery 100 stand at a test temperature of 25°C for 30 minutes; 2) Charge it to 4.5V with a constant current of 0.2C, and then discharge it to 0.02C with a constant voltage; 3) Let it stand for 5 minutes; 4) Charge it to 3V with a DC current of 0.2C and record the discharge capacity Q1 at 0.2C; 5) Let it stand for 5 minutes; 6) Charge it to 4.5V with a constant current of 0.2C, and then discharge it to the capacity cutoff of 0.02C; 7) Let it stand for 5 minutes; 8) Charge it to 3V with a DC current of 0.2C; 9) Plot a dv / dq curve for the charging capacity in step 6, draw a horizontal tangent at the cutoff voltage and read the corresponding abscissa, which is recorded as the capacity value Q. Then, calculate the CB value using the following formula: CB = Q / Q1.

[0059] As shown in Figures 3 and 4, in some embodiments, the width w of the recess 2110 is 70 μm to 100 μm, the depth h is 5 μm to 100 μm, and the center-to-center distance d between two adjacent recesses 2110 is 1 mm to 2 mm. Appropriately sized recesses 2110 can reduce the risk of excessive loss of negative electrode active material, which is beneficial for maintaining a high energy density and reducing the risk of lithium plating degradation within the recesses 2110. It can also suppress lithium ion accumulation within the recesses 2110 and maintain the structural stability of the negative electrode sheet 21 to reduce deformation of the electrode assembly 20. This further improves the problem of easy lithium plating within the recesses 2110 after the thick electrode sheet has recesses, thus improving the cycle performance of the secondary battery 100. In addition, an appropriate recess distribution density can improve electrolyte wetting, reduce ion migration resistance, and improve the problem of easy lithium plating within the recesses 2110 after the thick electrode sheet has recesses, thereby improving the cycle performance and kinetic performance of the secondary battery 100. The uniform distribution of the recesses 2110 can reduce the risk of localized overheating of the negative electrode 21. When gas is generated inside the secondary battery 100, it also facilitates the rapid discharge of gas from the electrode assembly 20, reducing the risk of pressure buildup inside the electrode assembly 20. This also improves the problem of lithium plating within the recesses 2110 and enhances the cycle performance of the secondary battery 100. Furthermore, by setting the relationship between the compaction density D and the total thickness T, this application can reduce the risk of sidewall collapse of the recesses 2110 even when their size or distribution density is small. The recesses 2110 can be formed using laser drilling, which offers high precision and facilitates the production of smaller and more uniformly distributed recesses 2110.

[0060] Referring to Figure 3, in some embodiments, the recess 2110 is square when viewed from the thickness direction X of the negative electrode 21 (i.e., the direction perpendicular to the paper in Figure 3). The square recess 2110 increases the contact area between the electrolyte and the negative electrode active material layer 211 at the boundary, which is beneficial for increasing the diffusion rate of lithium ions in the negative electrode active material layer 211, thereby further improving the problems of poor kinetic performance and low cycle capacity decay caused by the increased thickness of the negative electrode active material layer 211. Referring to Figure 4, the recess 2110 can also be circular when viewed from the thickness direction X of the negative electrode 21. The circular recess 2110 facilitates the uniform diffusion of lithium ions in the negative electrode active material layer 211, improves electrolyte wetting, and reduces ion migration resistance, thereby further improving the problems of poor kinetic performance and low cycle capacity decay caused by the increased thickness of the negative electrode active material layer 211.

[0061] Separating membrane

[0062] The separator 23 includes a membrane layer with a porous structure, and its material includes, but is not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the separator 23 may be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane, etc.

[0063] electrolyte

[0064] The electrolyte can be in one or more of the following states: gel, solid, and liquid. In some embodiments, the liquid electrolyte comprises a lithium salt and an organic solvent. The lithium salt may be selected from, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium tris(trifluoromethanesulfonyl)methyl lithium (LiC(SO2CF3)3), lithium dioxolaneborate (LiBOB), and lithium difluorophosphate (LiPO2F). 2) or one or more of the following. For example, LiPF6 is chosen as the lithium salt because it provides high ionic conductivity and improves cycling characteristics. The organic solvent can be a carbonate compound, a carboxylic acid ester compound, an ether compound, a nitrile compound, other organic solvents, or combinations thereof. Examples of carbonate compounds include, but are not limited to, diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), and fluoroethylene carbonate (FEC). 1,2-Difluoroethylene carbonate, 1,1-Difluoroethylene carbonate, 1,1,2-Trifluoroethylene carbonate, 1,1,2,2-Tetrafluoroethylene carbonate, 1-Fluoro-2-methylethylene carbonate, 1-Fluoro-1-methylethylene carbonate, 1,2-Difluoro-1-methylethylene carbonate, 1,1,2-Trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0065] In some embodiments, the concentration of the lithium salt in the electrolyte is from 0.5 mol / L to 1.5 mol / L. The lithium salt dissolves in the organic solvent and ionizes, partially forming solvated lithium ions and corresponding anionic groups, providing ion conductivity. Therefore, increasing the concentration of the lithium salt in the electrolyte is beneficial for improving the electrolyte conductivity, enhancing the kinetic performance of the secondary battery 100, and further reducing the possibility of lithium plating. The concentration of the lithium salt in the electrolyte can be determined using ion chromatography.

[0066] Referring to Figure 6, one embodiment of this application also provides an electronic device 1, which includes a battery compartment 101 and the aforementioned secondary battery 100. The electronic device 1 may include, but is not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors, etc.

[0067] The present application will be described below through specific embodiments and comparative examples. Those skilled in the art should understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0068] Example 1

[0069] (1) Preparation of positive electrode 22: Lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 96.5:1.5:2. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75wt%, and the mixture was stirred evenly. Foaming adhesive was first applied to a portion of the aluminum foil (positive current collector 220) with a thickness of 12μm. The slurry was then uniformly coated onto one surface of the aluminum foil. The mixture was heated to remove the foaming adhesive and expose a portion of the aluminum foil surface. The foil was then dried at 90°C to obtain a positive active material layer 221 with a coating thickness of 74μm. The above coating steps were repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode 22. The positive electrode 22 was then cold-pressed, and positive electrode tabs were welded onto the exposed aluminum foil. The positive electrode tabs were made of aluminum.

[0070] (2) Preparation of negative electrode sheet 21: The negative electrode active materials artificial graphite, conductive carbon black (Super P), and styrene-butadiene rubber (SBR) are mixed in a weight ratio of 96:1.5:2.5, and deionized water is added as a solvent to prepare a slurry with a weight percentage of 70 wt%, which is then stirred evenly. Foaming adhesive is first applied to the surface of the 8 μm negative electrode current collector 210, i.e., part of the copper foil. The slurry is then evenly coated on one surface of the copper foil, and heated to allow the foaming adhesive to fall off, exposing part of the aluminum foil surface. Then, it is dried at 110°C to obtain a negative electrode active material layer 211 with a coating thickness of 50 μm. The above coating steps are repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet 21, i.e., the total thickness T of the two negative electrode active material layers 211 is 100 μm. Then, the negative electrode active material layer 211 is cold-pressed. Multiple recesses 2110 are then formed on each negative electrode active material layer 211 using laser drilling. The width of each recess 2110 is 70 μm, the depth is 10 μm, and the center-to-center distance between two adjacent recesses 2110 is 1 mm. Then, negative electrode tabs are soldered onto the exposed copper foil. The negative electrode tabs are made of copper.

[0071] (3) Preparation of electrolyte: In a dry argon atmosphere, the organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are first mixed in a mass ratio of EC:EMC:DEC = 30:50:20. Then, lithium salt lithium hexafluorophosphate (LiPF6) is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0072] (4) Preparation of the isolation membrane 23: A polyethylene (PE) membrane with a thickness of 9 μm was selected.

[0073] (5) Assembly of the secondary battery 100: The positive electrode 22, the separator 23, and the negative electrode 21 are sequentially stacked and wound to obtain the electrode assembly 20. The dented aluminum-plastic film (150 μm thick) is placed in the assembly fixture with the dent facing upwards, and the electrode assembly 20 is placed in the dent. Electrolyte is injected into the dent of the aluminum-plastic film, and the positive and negative electrode tabs are led out of the aluminum-plastic film. Then, formation and encapsulation are performed to obtain the secondary battery 100 shown in Figure 1.

[0074] Examples 2-13 and Comparative Examples 1-2

[0075] The difference from Example 1 lies in the relationship between D and T.

[0076] Comparative Example 3

[0077] The difference from Example 1 is that the negative electrode active material layer 211 does not have a recess 2110.

[0078] Example 14

[0079] (1) Preparation of positive electrode 22: Lithium cobalt oxide (LiCoO2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 96.5:1.5:2. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%, and the mixture was stirred evenly. The slurry was uniformly coated on one surface of a 12 μm thick positive current collector 220, i.e., an aluminum foil, leaving an empty foil area at the edge of the aluminum foil. The foil was then dried at 90 °C to obtain a positive active material layer 221 with a coating thickness of 74 μm. The above coating steps were repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode 22. Then, the excess empty foil area was removed by laser die-cutting to obtain the positive electrode tab.

[0080] (2) Preparation of negative electrode sheet 21: Artificial graphite, conductive carbon black (Super P), and styrene-butadiene rubber (SBR) were mixed in a weight ratio of 96:1.5:2.5, with deionized water added as a solvent to prepare a slurry with a weight percentage of 70 wt%, and stirred evenly. The slurry was uniformly coated onto one surface of an 8 μm negative electrode current collector 210, i.e., a copper foil, leaving an empty foil area at the edge of the copper foil. It was then dried at 110°C to obtain a negative electrode active material layer 211 with a coating thickness of 65 μm. The above coating steps were repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet 21. The total thickness T of the two negative electrode active material layers 211 was then 130 μm. The negative electrode active material layer 211 was then cold-pressed. Multiple recesses 2110 are then formed on each negative electrode active material layer 211 using laser drilling. The width of each recess 2110 is 70 μm, the depth is 10 μm, and the center-to-center distance between two adjacent recesses 2110 is 1 mm. Then, the excess empty foil area is removed by laser die-cutting to obtain the negative electrode tab.

[0081] (3) Preparation of electrolyte: In a dry argon atmosphere, the organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are first mixed in a mass ratio of EC:EMC:DEC = 30:50:20. Then, lithium salt lithium hexafluorophosphate (LiPF6) is added to the organic solvent to dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0082] (4) Preparation of the isolation membrane 23: A polyethylene (PE) membrane with a thickness of 9 μm was selected.

[0083] (5) Assembly of the secondary battery 100: The positive electrode 22, the separator 23, and the negative electrode 21 are sequentially stacked to obtain a stacked electrode assembly 20. A 150μm thick aluminum-plastic film with a perforated surface is placed in the assembly fixture with the perforated surface facing upwards, and the electrode assembly 20 is placed in the perforation. Electrolyte is injected into the perforation of the aluminum-plastic film, and the positive and negative electrode tabs are led out of the aluminum-plastic film. Then, formation and encapsulation are performed to obtain the secondary battery 100 shown in Figure 5.

[0084] Comparative Examples 4-6

[0085] The difference from Example 14 lies in the relationship between D and T.

[0086] Ten secondary batteries from each comparative example and embodiment were taken for energy density, kinetic performance and cycle performance tests. The test results are recorded in Table 1.

[0087] The energy density test steps for the secondary battery are as follows: 1) Place the secondary battery at a test temperature of 25℃ for 30 minutes, charge it to 4.50V with a constant current of 0.7C, then charge it to 0.05C with a constant voltage of 4.50V, place it to stand for 5 minutes, discharge it to 3.0V with a constant current of 0.2C, and place it to stand for 5 minutes to obtain the discharge capacity C of the secondary battery; 2) After charging the above secondary battery to 3.95V with a constant current of 0.7C, charge it to 0.05C with a constant voltage of 3.95V, and then use a laser thickness gauge to measure the length L, width W, and height H of the secondary battery. The energy density (ED) = C / (L×W×H), with the unit being Wh / L. The average energy density of 10 samples is taken.

[0088] The kinetic performance of a secondary battery can be characterized by its low-temperature discharge performance. The test steps for low-temperature discharge performance are as follows: 1) Place the secondary battery at a test temperature of 25℃ for 5 minutes, then discharge it at a constant current of 0.2C to 3.0V, and let it stand for 5 minutes. This discharge capacity is taken as the initial discharge capacity; 2) Charge it at a constant current of 0.2C to 4.50V, then charge it at a constant voltage of 4.50V to 0.02C, and let it stand for 5 minutes; 3) Adjust the furnace temperature to -20℃, let it stand for 60 minutes, then discharge it at a constant current of 0.2C to 3.0V, and then let it stand for 5 minutes. The ratio of the discharge capacity of the secondary battery after low-temperature discharge to the initial capacity multiplied by 100% is the low-temperature capacity retention rate. The average low-temperature capacity retention rate of 10 samples is taken.

[0089] The cycle capacity retention test procedure for secondary batteries is as follows: 1) Place the secondary battery at a test temperature of 25℃ for 30 minutes, charge it to 4.43V with a constant current of 1C, then charge it to 0.05C with a constant voltage, let it stand for 5 minutes, and then discharge it to 3.0V at 0.7C. This discharge capacity is the initial discharge capacity and is counted as 100%; 2) Cycle the battery 1000 times according to the above charge and discharge procedure. The ratio of the discharge capacity to the initial capacity after the cycle is multiplied by 100% to obtain the capacity retention rate. The average value of the capacity retention rate of 10 samples is taken.

[0090] The lithium plating test procedure for secondary batteries is as follows: 1) Charge the secondary battery after cycle testing to 4.43V at a constant current of 1C, and then charge it to 0.05C at a constant voltage; 2) Disassemble the secondary battery and check whether lithium plating has occurred in the recessed area of ​​the negative electrode. If the lithium plating area is greater than or equal to 2mm², the lithium plating area will be considered as lithium plating. 2 If the lithium plating test fails, it is considered lithium plating and the test is deemed unsuccessful. The percentage of samples that pass the lithium plating test out of 10 samples is the lithium plating test pass rate. The results are recorded in Table 1.

[0091] Table 1

[0092] As shown in Table 1, for the wound electrode assembly, compared to the thick electrode sheets with recesses in Comparative Examples 1-2, Examples 1-11, by setting the recesses and ensuring that the compaction density D and total thickness T satisfy 3T+1≤D≤8T+1, allow the secondary battery to maintain a high energy density while improving the ion transport path, thus increasing the low-temperature capacity retention rate and improving the electrolyte wetting, thereby improving the lithium plating test pass rate and the cycle capacity retention rate. Therefore, the secondary batteries of Examples 1-11 can achieve both high energy density and excellent low-temperature capacity retention and cycle capacity retention. Although the compaction density D and total thickness T of Comparative Example 3 satisfy the above relationship, no recesses are provided, resulting in poor ion transport path and electrolyte wetting, leading to lower low-temperature capacity retention and cycle capacity retention rates. Furthermore, Examples 2-8 satisfy 3.5T+1≤D≤6.5T+1, which can further improve the energy density, low-temperature capacity retention rate, and cycle capacity retention rate of the secondary battery. Furthermore, Examples 4-6 satisfy 4.5T+1≤D≤5.5T+1, which can further improve the energy density, low-temperature capacity retention rate, and cycle capacity retention rate of the secondary battery.

[0093] Similarly, for the stacked electrode assembly, compared to the thick electrodes with recesses in Comparative Examples 4-5, Example 14, by setting both the recess and the compaction density D and total thickness T to meet specific conditions, allows the secondary battery to achieve both higher energy density and better low-temperature capacity retention and cycle capacity retention. While the compaction density D and total thickness T of Comparative Example 6 meet the above relationship, the absence of recesses results in poor ion transport paths and electrolyte wettability, leading to lower low-temperature capacity retention and cycle capacity retention in the secondary battery.

[0094] Furthermore, the compaction density of Examples 1-14 meets the requirement of 1.2 g / cm³. 3 ≤D≤2.2g / cm 3 At that time, secondary batteries can achieve both high energy density and better low-temperature capacity retention and cycle capacity retention.

[0095] Examples 15-18

[0096] The difference from Example 1 is that in the wound electrode assembly 20, the compaction density D1 of the first active material region 2111 located in the straight section 20A is different from the compaction density D2 of the second active material region 2112 located in the curved section 20B.

[0097] Examples 19-22

[0098] The difference from Example 14 is that the compaction density D3 of the third active material region 2113 located in the central region is different from the compaction density D4 of the fourth active material region 2114 located in the edge region.

[0099] Table 2

[0100] As shown in Table 2, for the wound electrode assembly, compared to Example 1, the first active material region in Examples 15-17 has a higher compaction density than the second active material region. While the lithium plating throughput, low-temperature capacity retention, and cycle capacity retention of the secondary battery did not decrease significantly, the energy density was significantly improved. Compared to Example 18, Examples 15-17 reduced the impact of the first active material region's compaction density on the lithium plating throughput, low-temperature capacity retention, and cycle capacity retention by limiting the upper limit of the compaction density.

[0101] Compared to Example 14, for the stacked electrode assembly, the third active material region in Examples 19-21 has a higher compaction density than the fourth active material region. While the lithium plating throughput, low-temperature capacity retention, and cycle capacity retention of the secondary battery are not significantly reduced, the energy density is significantly improved. Compared to Example 22, Examples 19-21 reduce the impact of the third active material region's compaction density on lithium plating throughput, low-temperature capacity retention, and cycle capacity retention by limiting the upper limit of this compaction density.

[0102] Examples 23-37

[0103] The difference from Embodiment 1 lies in the relevant dimensions of the recess 2110.

[0104] Table 3

[0105] As shown in Table 3, compared to Examples 26 and 32, the appropriately increased size of the recesses in Examples 1, 23-25, and 28-31 improves the wetting degree of the electrolyte in the entire negative electrode active material layer and reduces ion migration resistance. Therefore, the lithium plating throughput of the secondary battery is improved, as are the cycle capacity retention rate and low-temperature cycle retention rate. Compared to Examples 27 and 33, Examples 1, 23-25, and 28-31 limit the upper limit of the recess size, avoiding the loss of more negative electrode active material and reducing the risk of side reactions caused by increased contact area between the electrolyte and negative electrode active materials. This reduces impedance, allowing the secondary battery to maintain a higher energy density, and also improving the lithium plating throughput and cycle capacity retention rate.

[0106] Compared to Example 36, the distribution density of the recesses in Examples 1 and 34-35 is appropriately increased, avoiding the loss of more negative electrode active material and reducing the risk of side reactions caused by increased contact area between the electrolyte and negative electrode active materials, thus lowering impedance. Therefore, the secondary battery can maintain a higher energy density, and the lithium plating throughput and cycle capacity retention are also improved. Compared to Example 37, the upper limit of the recess distribution density is limited in Examples 1 and 34-35, thus increasing the wetting degree of the electrolyte in the entire negative electrode active material layer and reducing ion migration impedance. Therefore, the lithium plating throughput of the secondary battery is improved, and the cycle capacity retention and low-temperature cycle retention are also improved.

[0107] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Therefore, any equivalent variations made in accordance with this application are still within the scope of this application.

Claims

1. A secondary battery, comprising an electrode assembly, wherein the electrode assembly includes a negative electrode sheet, wherein, The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector. The negative current collector includes a first region, and the negative active material layer is disposed on both opposite surfaces of the first region. The total thickness of the negative active material layer on the first region is T, where 0.07 mm ≤ T ≤ 0.15 mm. The negative electrode active material layer has multiple recesses, and the compaction density of the negative electrode active material layer is D, with units of g / cm³. 3 , 3T+1≤D≤8T+1.

2. The secondary battery as described in claim 1, wherein, 3.5T+1≤D≤6.5T+1.

3. The secondary battery as described in claim 2, wherein, 4.5T+1≤D≤5.5T+1.

4. The secondary battery according to any one of claims 1 to 3, wherein, 1.2g / cm 3 ≤D≤2.2g / cm 3 。 5. The secondary battery as described in claim 4, wherein, 1.3g / cm 3 ≤D≤1.9g / cm 3 。 6. The secondary battery as described in any one of claims 1 to 3, wherein, The electrode assembly has a wound structure, comprising a straight section and a curved section connected along the winding direction. The negative electrode active material layer includes a first active material region located in the straight section and a second active material region located in the curved section. The compaction density of the first active material region is D1, and the compaction density of the second active material region is D2, 1.3 g / cm³. 3 ≤D1≤2.0g / cm 3 1.2g / cm 3 ≤D2≤1.5g / cm 3 .

7. The secondary battery as claimed in any one of claims 1 to 3, wherein, The electrode assembly has a stacked structure. The negative electrode active material layer includes a third active material region and a fourth active material region disposed around the outer periphery of the third active material region. The compaction density of the third active material region is D3, and the compaction density of the fourth active material region is D4, 1.3 g / cm³. 3 ≤D3≤2.2g / cm 3 1.2g / cm 3 ≤D4≤1.7g / cm 3 .

8. The secondary battery according to any one of claims 1 to 7, wherein, The width of the recess is 70μm to 100μm, the depth is 5μm to 100μm, and the center-to-center distance between two adjacent recesses is 1mm to 2mm.

9. The secondary battery according to any one of claims 1 to 8, wherein, Viewed from the thickness direction of the negative electrode sheet, the recess is circular or square.

10. The secondary battery according to any one of claims 1 to 9, wherein, The CB value of the secondary battery is 1 to 1.

05.

11. The secondary battery according to any one of claims 1 to 9, wherein, The recess is obtained by removing part of the negative electrode active material layer.

12. An electronic device, wherein, It includes a battery compartment and a secondary battery as described in any one of claims 1 to 11 disposed within the battery compartment.

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